Align the spec (and write it) and the checks for `@targetName`

[hamzaremmal]

Compiler version

312c89a

Minimized example

@annotation.targetName("foo" + "bar") def foo = ???

Output

-- Error: ----------------------------------------------------------------------
1 |@annotation.targetName("foo" + "bar") def foo = ???
  |                       ^^^^^^^^^^^^^
  |                       @targetName needs a string literal as argument
1 error found

Expectation

Should compile. The spec says (should say actually) that the parameter of an @targetName annotation should be a constant expression.